The present invention relates to a method for improving the properties of stretch blow molded thermoplastic containers, especially polyethylene terephthalate (PET), by heat treating to induce selective crystallization thereof.
It is known that the gas transmission impedance and the mechanical properties of PET improve with increasing degree of crystallization. The same properties are also improved by orientation, albeit to a different degree.
Crystallites forming from the amorphous phase of unoriented PET, as by heating, are relatively large spherules, while crystallites forming due to orientation or in oriented PET are finely dispersed. The index of light refraction of the large-spherulitic structure being different from that of the optically clear, amorphous matrix, opacity results. This is not the case for the finely dispersed phase and therefore the oriented PET articles remain clear in spite of high levels of crystallinity induced in them after orientation. Mechanically induced crystallization due to deformation is limited by the fact that PET embrittles as it crystallizes and therefore the deformation cannot be continued past the amount at which the brittleness is sufficient to preclude further deformation. As a practical matter, the amount of crystallinity in a container made according to commercial practice is usually between 18% and 25%. The resulting finely dispersed crystallites do not change effective transparency and, since they serve as nuclei in the course of subsequent additional crystallization that may be induced by heat treatment, such subsequently formed crystallites are finely dispersed as well. The present invention provides an efficient way to render PET containers transparent and haze-free in spite of their being substantially crystallized by a process that insures a high degree of orientation, e.g., by stretch-blow molding, and heat treating to crystallize.
PET containers for certain commodities are filled hot and they must be "heat-set". Crystallization is a by-product of heat setting, an operation intended to relieve the residual stresses within the biaxially oriented PET containers so that they may retain their shape when exposed to the temperatures of filling which are higher than the ones at which they were formed. This is accomplished by stress-relieving them at a temperature that is higher than the one to which they may be subsequently exposed, i.e., heat-setting. In most instances, that temperature is within the range capable of propagating crystallization and therefore, typically, heat-set, blow molded PET containers are crystallized beyond the amount due to deformation alone. Typical U.S. patents which illustrate this practice are U.S. Pat. Nos. 4,871,507, 4,913,945, 4,803,036 and 4,318,882.
Crystallization proceeds in PET at a rate corresponding to the temperatures at which it is induced, and quite rapidly at higher temperatures of heat setting. Also, crystallization of an amorphous matrix occurs during its mechanical deformation, as by stretch blowing of a largely amorphous preform into a container or bottle.
Given the practical limits of the amount of deformation (the degree of orientation) that can be imparted to a preform, thereby crystallizing some 25% of the original amorphous phase at best, heat-treatment must be resorted to if higher crystallinity, e.g., near 50% is to be obtained with the temperature as high as practicable, but, of course, below the melting point, and maintained for as long as economical production practice permits.
The rate of crystallization depends not only on time and temperature, but also on the amount of amorphous matrix that is already crystallized at a given moment. As a practical matter, some 45% or slightly more of the matrix may be crystallized within a reasonable processing time of a minute or less when starting with the level of crystallinity inherently present in a typical stretch-blown PET container.
The crystalline phase is considerably less pervious to gases than the amorphous phase. Also, gases dissolve less in the crystalline phase than in the amorphous phase and at a slower rate. The crystalline regions are denser than the amorphous ones and are stronger, but also brittler. In combination, these properties can provide a superior container to retain or exclude fluids, to avoid dissolving them in the container walls and to reinforce the structure. The quality of a container in these respects is measured in terms of its shelf-life, i.e., the amount of a specified time before it fails to preserve the integrity of its contents under normal conditions of use.
For example, in the case of a PET bottle for carbonated beverages, the shelf-life is largely given by the net loss of CO.sub.2, which is the sum of losses due to transport through the bottle walls, the amount dissolved therein, and its escape into the space above the liquid level (the head space), due to its enlargement, i.e., creep under prolonged pressure, often aggravated by elevated ambient temperatures. Increased crystallinity will alleviate all three of these harmful effects.
However, transparency of the container is an important property of PET when used for beverage bottles, because the public finds it appealing, the same as in glass which PET replaces. Transparency is affected by crystallization, as indicated hereinabove.
Thus, when crystallizing the amorphous phase without having imposed thereon a certain molecular arrangement (as by deformation), comparatively large spherules will form which defract light and which reduce or eliminate transparency. The small, finely dispersed crystallites that form during deformation do not interfere with light transmission. The same is true of crystallites that form in addition to those already present due to deformation. Therefore, oriented parts of a bottle may be thermally crystallized close to 50% without significant loss of transparency, while the other parts will lose transparency in the decreasing amount of the deformation they have experienced.
It is important, therefore, to carefully balance the crystallinity and strength in the various portions of the container. A bottle must be clear where its contents can be seen, and it also must be impervious to CO.sub.2 over a sufficient part of its surface to provide acceptable shelf-like, permeation being a function of the area. It must also exhibit creep resistance, and have high impact resistance.
The art shows no practical procedure to obtain a satisfactory balance of these properties. U.S. Pat. No. 4,233,022 shows a method for heat setting which obtains a limited crystalline morphology in stretch blow molded bottles through heating subsequent to blow molding by conduction in a confining mold, which is time consuming, inconvenient and above all, not suitable to control the process with the necessary precision. To heat the container so as to match the resulting amount of crystallinity against the amount of deformation its several portions have undergone is not possible with a device, such as a mold, that transfers heat into the container by conduction. Design and fabricating limitations of a mold allow for no more than comparatively coarse definition of heating zones, even if the best care is taken, for example, to insulate sections of the mold that are individually heated as by a circulating fluid in the normal manner, as taught in U.S. Pat. No. 4,233,022. This is particularly evident at transition zones from the body to the neck over the shoulder, or from the body downward into the base proper. In these regions, orientation changes from a maximum to near zero, and increasing haze develops unless heating is accurately aimed and its intensity appropriately metered. To accomplish this with a zone-controlled mold is difficult at best, and even at limited levels of effectiveness expensive and time consuming.
Heating by radiant or aimed convective heat transfer is preferable, because it can be accurately metered according to preselected zones and locations. However, neither radiant, nor convective heating is possible in the confined environment of a mold, which was partly considered to be necessary according to the art because, it was found, the distortion of the oriented article upon heating above the temperature of orientation may be counteracted by the application of internal pressure, so long as that temperature is no more than enough for heat setting.
The above distortion occurs for two reasons, the relief of residual stresses in the oriented article, and shrinkage due to crystallization. Residual stresses arise, as is well known, in the course of forming the article in a mold that precludes free thermal shrinkage as said article cools from the temperature at which it is formed to that of the mold in which it is formed. Whenever the same article is once again heated, to or above said temperature, but without confinement, the residual stresses are relieved and the shape of the article changes. Shrinkage due to crystallization is of a different nature: the crystalline phase is denser than the amorphous one and therefore, the dimensions of the article will be reduced while it crystallizes. Thus, it is known that density of amorphous PET=1.333 g/cm.sup.3, while that of a crystal unit cell is calculated to =1.455 g/cm.sup.3 (Ref. U.S. Pat. No. 2,968,065). In a given article, the actual shrinkage of its specific volume will be between these limiting values, i.e. appr. 9% max. and 0%, depending on the degree of crystallization.
Accordingly, in a typical case, the total distortion does not set in at the same time. Instead, stress relief occurs upon heating first, which changes the shape, but not the density and therefore the specific volume, and only then does the distortion due to shrinkage of that volume set in, the more as crystallization progresses.
The change in shape due to stress relief, in contrast with that due to increased density (crystallinity), is readily corrected, as by re-blowing in a mold. The change in shape due to crystallization may be corrected as a practical matter only within certain limits, i.e. the specific volume of the original article may be restored, but only at some sacrifice of its original shape.
In current practice, which is largely confined to heat-setting, as above explained, the dominant effect of heating is stress relief, while only modest crystallization occurs. In contrast, the aim here is maximum crystallization, the deformation of which cannot be offset as before.
Accordingly, it is found that heating under pressure is no benefit, but a hindrance, and that it becomes possible to use radiant or convective heat transfer, which is far more efficient and controllable than conductive heating. Radiant heating, which is preferred, is usually accomplished by means of electric resistance heating rods. Convective heating may be effected by means of a heated gas aimed at the container from orifices spaced according to a suitable pattern. Naturally, for effective radiant or convective heating, the article to be heated must be kept unconfined, at a predetermined distance from the heating means. Nor is it useful, in the case of a container, to keep it under internal pressure, which is necessary in the case of conductive heating by means of a mold.
To obtain the final shape and predetermined contained volume of the oriented container. The heated and thereby distorted shape is re-shaped, usually re-blown in conventional ways, using a suitable fixture.
It is, therefore, a principal object of the present invention to provide an efficient process for heat-treating thermoplastic containers, especially of polyethylene terephthalate (PET), wherein stretch blow molded containers are selectively crystallized according to the amount of orientation of its individual regions while preserving substantial transparency throughout.
It is a still further object of the present invention to provide a process as aforesaid which is effective, expeditious and convenient to use on a commercial scale.
It is a still further object of the present invention to provide a process as aforesaid which obtains improved containers having greatly improved properties.
Further objects and advantages of the present invention will appear hereinbelow.